Fluidized combustor

ABSTRACT

A system and method to permit fluidized combustion without requiring a separate fluidizing medium, such as sand. The system may include a combustion chamber, a fuel feed system coupled to the combustion chamber, an ignition system coupled to the combustion chamber, a fluidizing system coupled to the combustion chamber, where the fluidizing system includes a fuel and fluidizing medium formed from the same material, and a heat recovery system. The method may include delivering a pelletized fuel into a combustion chamber by a fuel feed system and onto a fluidizing plate, igniting the pelletized fuel with an igniter system coupled to the fluidizing plate, fluidizing the pelletized fuel after combustion is started in the absence of a fluidizing medium beyond the pelletized fuel, and heating a medium in a heat exchanger with combustion gases.

PRIORITY

This application claims the priority benefit of U.S. Provisional Patent Application No. 61/250,843, filed Oct. 12, 2009, which is incorporated by reference in its entirety into this application.

BACKGROUND

Combustion provides a heat source to use in many applications, including stoves, central heating furnaces, and other heating appliances. Various combustions systems currently exist. For example, a fire tube where the combustion fire occurs within a tube, which heats water within an outer shell. Alternatively, the boiler may be a tube containing water that is heated on an outer surface. Generally the fire tube designs are for smaller applications, while the water tube provides for larger applications, as it can be configured for higher pressures.

One combustion system uses wood pellets as the combustion fuel, as opposed to fossil fuels. Wood pellets are a type of wood fuel generally made from compacted sawdust, the byproduct of sawmilling and other activities involving wood. The pellets may be made at a very high density and low humidity content that allows for very high combustion efficiency.

Currently, existing wood pellet combustion systems use ‘pile burning’ or grate burning’ techniques. These combustors generally ignite the fuel with an ignition system, and then remove the ash and burned byproducts with a rake system either during or after use. Another technique commonly used in Europe on small residential systems is a horizontal flow combustor. These systems either require maintenance while the operator is subjected to the heat of combustion, or they require that the ash is removed during a shutdown period. A raking mechanism may be used to remove ash from underneath the combustion space. The raking technique has significant disadvantages for the operator. The igniter may also need to be removed from the combustion system during full operation to reduce exposure to the heat of combustion.

Another combustion system includes fuel fluidized bed combustion (FBC) systems. FBC is a combustion technology generally used in power plants. Fluidized beds suspend fuels on upward-blowing jets of air during the combustion process. Generally, a gas or liquid fluid is forced up through a granular solid material, i.e. ‘a fluidizing medium’ such as sand, with enough force to suspend the fuels and solids and cause it to behave as though it were a fluid. The resulting turbulent mixing of fuel, gas, and fluidizing medium provides more effective chemical reactions and heat transfer. Most fluidized boilers require separate ‘fluidizing medium,’ such as sand, which helps to break down the fuel. However, a large amount of energy may be required to fluidize the sand.

BRIEF SUMMARY

A combustion system for use with a pelletized fuel in the absence of a separate fluidizing medium is described, including a combustion chamber, a fuel feed system coupled to the combustion chamber, an ignition system coupled to the combustion chamber, a fluidizing system coupled to the combustion chamber, wherein the fluidizing system includes a fuel and fluidizing medium formed from the same material; and a heat recovery system. In operation, the pelletized fuel agitates other fuel pellets and acts as the fluidizing medium. Therefore, a separate fluidizing medium, such as sand, is not required, thereby increasing the efficiency of the combustion system.

Embodiments of the fluidizing system may include a fluidizing plate at a bottom of the combustion chamber with a plurality of holes to distribute air over the combustion chamber from a primary air system coupled to the combustion chamber through the fluidizing plate. The primary air system may supply air to the combustion chamber by a blower and manifold through the fluidizing plate. The fluidizing system may further include a secondary air system to introduce air into the combustion chamber over the fluidizing plate, where the secondary air system introduces air into the combustion chamber off a radial axis of the combustion chamber to promote swirling of combustion gases. In a preferred embodiment, the secondary air system introduces the air over the fluidizing plate, but under the heat recovery system and into the combustion chamber substantially tangential to the circumferential edge of the combustion chamber and substantially perpendicular to a longitudinal axis of the combustion chamber to provide a substantially circumferential air flow into the combustion chamber.

Embodiments of the fuel feed system includes an entry port common to the secondary air system. The secondary air system may introduce air into the combustion chamber at a speed greater than a flame propagation speed to prevent combustion of incoming fuel. The fuel feed system may also include a fuel bin and fuel transport, which introduces the pelletized fuel into the combustion chamber along the fuel transport at a non-zero angle from horizontal. For example, the fuel is transported at an angle of between 15 and 45 degrees below horizontal from the entry port, and preferably at an angle of approximately 25 degrees to prevent hot combustion gases from entering the fuel feed system.

Embodiments of the ignition system may include an igniter on a first side of the fluidizing plate and a fin on a second side of the fluidizing plate coupled to the igniter through the fluidizing plate, wherein the first side of the fluidizing plate faces away from the combustion chamber. The fluidizing plate may be composed of a heat insulating material, such as stainless steel, and the fin may be composed of a heat transfer material, such as carbon steel. The igniter on the first side of the fluidizing plate away from the combustion zone is therefore protected by the heat insulation of the fluidizing plate while permitting sufficient heat to enter the combustion zone through the heat transfer through the fin. The igniter may further be positioned relative to the fluidizing plate to be cooled by air entering the combustion chamber through the fluidizing plate. The igniter may also include a pulse cartridge to provide heat to the fin in pulses and reduce the chances of overheating the igniter.

In an exemplary embodiment, the combustion chamber is coupled to the combustion system by a hinge to permit access to an interior of the combustion chamber and ignition system. Preferably the pelletized fuel is wood pellets to permit fluidized combustion without requiring a separate fluidizing medium, such as sand, to abrasively agitate the fuel during combustion.

Embodiments of the heat recovery system include a heating device positioned over the combustion chamber to share a common interior space, the heating device including at least three surfaces positioned so that combustion gases contact a first surface of the heating device at a first pass, contact a second surface of the heating device during a second pass, and contact a third surface of the heating device during a third pass; wherein the first surface is an interior surface of the heating device facing the common interior space, and the second and third surfaces are substantially vertical so that the second pass and third pass occur in substantially opposite directions. The combustion gases are directed substantially downward, toward the Earth in the second pass, and substantially upward, away from the Earth, in the third pass. The change in direction and the temperature variation between the second and third pass permit particulate within the combustion gasses to gravity-drop out of the gas stream into a collection stream providing a cleaner exhaust gas.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1B illustrate an exemplary combustion system according to embodiments as described herein.

FIGS. 2A-D illustrate various views of an exemplary igniter system plate according to embodiments described herein.

DETAILED DESCRIPTION

The following detailed description illustrates by way of example, not by way of limitation, the principles of the invention. This description will clearly enable one skilled in the art to make and use the invention, and describes several embodiments, adaptations, variations, alternatives and uses of the invention, including what is presently believed to be the best mode of carrying out the invention. It should be understood that the drawings are diagrammatic and schematic representations of exemplary embodiments of the invention, and do not limit the present invention nor are they necessarily drawn to scale. Although embodiments of the invention may be described and illustrated herein in terms of a wood pellet combustion system, it should be understood that embodiments of this invention are not so limited, but are additionally applicable to other biomass fuel sources. For example, alternate fuel sources may include nut shells (such as almond, hazelnut, or peanut), seeds, switchgrass, and corn husks. These materials may be provided whole, chopped, ground, or pelletized. For example, whole or crushed nut shells and seeds may be used, or pelletized switchgrass. Other biomass fuel sources may also benefit from the invention as described herein, and are within the scope of the present invention.

An exemplary combustion system includes a chamber for the mixing a fuel source and air. In one embodiment, the combustion system is in a vertical up arrangement to permit substantially complete combustion within the furnace without needing to remove ash from within the combustion space. Below the combustion space is a fluidizing plate that permits air to flow into the combustion space. A primary air system may be used to inject air through the fluidizing plate into the combustion space. An igniter may be coupled to the fluidizing plate to ignite the pellets and permit combustion. The system may additionally include a secondary air system to inject air above the fluidization zone, increasing air mixing and improving combustion efficiency.

In one embodiment, the combustion fuel is wood pellets. Air may be injected from the primary air system, through the fluidizing plate, to mix with the wood pellets. Preferably, sufficient air velocity is used to suspend and mix the wood pellets with the injected air. With other FBC systems, sand may be used to assist in the chemical reactions of the fuel combustion and heat transfer. Embodiments of the present combustion system do not require sand, as the bubbling of the fluidized wood pellets may cause them to break down as they rub against each other in the presence of heat from combustion. Therefore, high combustion efficiency may be attained without the significant energy required to fluidize sand.

FIGS. 1A and 1B illustrate an exemplary combustion system according to embodiments as described herein. Embodiments of the combustion system 10 use a fluidized fuel combustion process to provide very high combustion efficiencies. Referring to FIG. 1A, the exemplary combustion system includes a combustion chamber 2 a, heat exchange chamber 2 b, an emissions system 14 for the output of combustion gases, and a control system 16 to control the combustion system 10. At or near the bottom of the combustion chamber 2 is an igniter system 4 according to embodiments as described herein. In one embodiment, the igniter system includes a fluidizing plate and heat distributors. The system may include one or more air systems 6, 8 to fluidize the fuel. For example, a primary air system 6 may be below the igniter system, while a secondary air system 8 may be over the combustion area. A fuel feed system 12 may also be included to deliver the fuel to the combustion chamber 2.

The exemplary combustion system 10 includes a fuel feed system 12. In one embodiment, the system includes a fuel feed bin 18, a motor 20, and an auger 22. The fuel feed bin 18 permits storage of an amount of fuel to be used during the combustion cycle. The bin may be of various sizes depending on the application and desired running time. The bin may further be coupled by a second auger or other conveyor to an outside larger storage bin (not shown). The feed bin of the system may then directly feed the combustion system during use for a set amount of time, but may be refilled by the larger storage bin remote from the system (manually, or automatically through auger or conveyer, etc.). Limiting the size of the fuel feed bin 18 may reduce potential fire hazards, in the case of uncontrolled combustion. During operation, the fuel feed bin 18 may drop a metered amount of fuel onto the auger 22 driven by the motor 20. The motor 20 may be a rotary gear motor coupled to the fuel feed auger 22 to provide a metered amount of fuel to the combustion chamber 2 b by operating over a variable rpm range, depending upon the combustion load.

In one embodiment, the fuel is conveyed to the combustion chamber at an incline. By inclining the fuel entry port, the chance of combustion gases entering the fuel delivery system may be minimized. For example, the pelletized fuel is introduced into the combustion chamber along the auger 22 at a non-zero angle from horizontal. The auger 22 may be included above or below horizontal, such that the feedstock is transported along an upward incline or along a downward path to the entry port. In the embodiment shown, the delivery system is inclined upward toward the top of the combustion chamber. The exact location and orientation of the fuel feed system 12 and entry port into the combustion chamber may be chosen based on size and space constraints of the system. For example, the fuel feed auger 22 is inclined from a horizontal reference line at an angle, θ, between approximately 15 and 45 degrees, i.e. the feed auger is inclined up into the combustion tube by about 15 to 45 degrees. More specifically, the auger is angled by approximately 25 degrees.

The fuel is then delivered to the combustion chamber 2 through a port 24 in the side of the combustion chamber. The port 24 is located above the igniter system 4. Preferably, the port 24 is as high as possible over the combustion chamber 2. However, the port 24 is also preferably below the heat exchange chamber 2 b to reduce interference with the heating device and ease manufacturing. This permits the fuel to drop either onto the igniter system 4 or into the combustion zone of the combustion chamber 2 a during operation and reduces interference with the heating element.

FIGS. 2A-2D illustrates an exemplary fluidizing plate and igniter system according to embodiments described herein. At start up, the fuel drops under gravity onto the ignition system. In one embodiment, the ignition system is coupled to the fluidizing plate. An electric igniter may be used to ignite the fuel during start up of the combustion along with a cartridge heater to indirectly ignite the fuel by heat transfer, which gives the cartridge heater a longer service life. In an exemplary embodiment, the ignition system includes an electric heating element on a lower side of the fluidizing plate, with fins that extend into the combustion chamber on an opposite side of the fluidizing plate, to heat the fuel through resistance heating.

FIG. 2A illustrates a top view of one embodiment of the fluidizing plate 26 with a grid of air distribution holes 30. The holes 30 may be sized to permit a sufficient velocity of air to pass to fluidize the combustion fuel. Preferably the holes 30 are sized to prevent the combustion fuel from passing below the fluidizing plate 26. Under fire, air passes through a specifically sized fluidization plate to permit a sufficient velocity so that fluidization of the combustion fuel occurs.

FIGS. 2B and 2C illustrate one embodiment of an igniter 28 including an electric heating element. The electric heating element, such as resistance heater 28 a, is coupled to the fluidizing plate 26. The heating element 28 may be a resistance heater 28 a in direct contact with the fluidizing plate 26. A portion of the resistance heater, such as a coupled plate 28 b, may pass through the fluidizing plate 26 and enter the combustion space. The coupled plate 28 b may be mounted to the fluidizing plate 26, such as for example by press fit. An electric resistance heater 28 a is coupled to the underside of the fluidizing plate, away from the combustion space. The fluidizing air may be used to cool the igniter once combustion has been initiated and is sustainable. The fluidizing air may protect the igniter so that the igniter does not need to be removed while the combustion system is in full operation. A control signal may also be used to establish the amount of heat and length of time required to ignite the combustion fuel to sustainable combustion. The combustion side of the fluidizing plate may reach temperatures around 2400 degrees Fahrenheit during sustained combustion, while the under air side of the fluidizing plate, away from the combustion space, cooled by the under bed air may remain at temperatures below 1500 degrees Fahrenheit. Therefore, removing the igniter from the combustion chamber may increase working life.

In one embodiment, the igniter is a cartridge heater that pulses heat to the fuel to further reduce the potential for over-heating. For example, the heating element may include a pulsing cartridge to prevent or reduce overheating the igniter during start up. Resistance heaters then contact the fuel and provide focused heat. The fuel in contact with the plates is heated and begins to smolder and burn. Removing the igniter from inside the combustion chamber extends the igniter life as it is not subjected to the extreme continuous heat associated with sustained combustion.

FIG. 2D illustrates a portion of the igniter system 4 in an exploded three dimensional view, including the heating element 28 components composed of a resistance heater 28 a, plates 28 b coupled to fluidizing plate 26 with fluidizing air holds 30. As illustrated, the resistance heater includes a resistance heater housing 28 a coupled to a plate 28 b that may penetrate the fluidizing plate 26 and enter the combustion space. In one embodiment, the fluidizing plate is composed of a heat insulating material, while the heating element 28 is of a high heat transfer material. For example, a combination of carbon and stainless steel alloys may be used for the fluidizing plate and ignition system to create sufficient heat transfer from the electric resistance heater to the wood pellets for ignition but permit sufficient cooling of the heater by the fluidizing air during operation. In one embodiment, stainless steel is used as an isolator while the carbon steel is used as the conductor. Carbon steel is used to house the igniter and continues from the igniter housing through the fluidizing plate to allow sufficient heat transfer directly to the combustion fuel without subjecting the igniter to the heat of sustained combustion, prolonging the life of the heating element. The fluidizing plate is of stainless steel to insulate the igniter from the heat of combustion, once the ignition of the fuel is complete.

Referring back to FIG. 1A, the exemplary combustion system 10 may include a primary air system 6. The primary air system 6 directs air up through the igniter system 4 and into the combustion chamber 2 a. The primary air system 4 may include a fan 32 located away from the combustion chamber 2 a, while the air is directed by a manifold 34. The primary air fan 32 may be a high pressure, such as over 15 inches water column, low volume blower to provide high velocity under bed air to fluidize the combusted fuel. The manifold may uniformly deliver the primary combustion air to the distribution fluidizing plate 26. The fluidizing plate 26 then distributes the primary air evenly over the combustion region, and also provides an increase in air velocity to fluidize the combusted fuel. During start up, the primary air fan 32 may be used to provide sufficient oxygen to start combustion. For instance, the fan 32 is set to a lower level not sufficient to fluidize the fuel material; else the air may cool the fuel before combustion is sustainable. The primary air system 6 may be used to cool the igniter 4 during start up, and during sustained combustion.

The combustion system 10 may also include a secondary air system 8. The secondary air may be injected above the fluidization zone, increasing air mixing to improve combustion efficiency. In one embodiment, the combustion system 10 includes a secondary air system 8 that enters the combustion chamber 2 a over the combustion area. The secondary air system 8 may include a fan 36 and manifold 38 to direct the secondary air flow. The fan 36 may be a high volume, low pressure fan, such as under 15 inches water column, that provides combustion air above the fluidized fuel zone. The secondary air system 8 allows additional air to mix with the combustion gas stream inside the combustion chamber 2 a. For example, the primary air system 6 provides approximately 60% of the oxygen needed for sustained combustion, while the secondary air system 8 provides the remaining oxygen. The secondary air system 8 also helps mixing for more efficient and complete combustion. The secondary air system 8 may also be used to add additional air to the exiting combustion gases in order to dilute the exit gases and lower emissions ratios.

In the exemplary combustion system shown, the secondary air system 8 enters the combustion chamber 2 a at the same port 24 as the fuel feel system 12. Preferably, the port 24 is directed off the radial axis of the combustion chamber. This directs the secondary air into the combustion chamber off axis and promotes mixing. Preferably, the port 24 is directed substantially tangential to the edge of the combustion chamber to provide a swirling effect to the fluidized fuel within the combustion chamber. The air and fuel port is configured to combine the entry of the over fire air with the feed inlet. The configuration provides an air lock, allowing fuel to enter the combustor without hot combustion gasses entering the fuel feeding and storage systems. The secondary air system 8 provides sufficient velocity to the secondary air to overcome the flame propagation speed and localize the combustion within the combustion chamber.

During operation, the combustion gases rise to the top of the combustion chamber and through heat exchange chamber 2 b, following generally path 40 a, of FIG. 1A. The hot gases, in a first pass 40 a, transfer heat to the heat exchange system 42. The combustion gases are directed down over the top of the combustion chamber for a second pass 40 b of the heat exchange system 42. The second pass 40 b cools the gases as the heat is transferred to the heat exchange system 42. Once the temperature of the combustion gas drops, unburned organic and inorganic particles from the combustion process drop out of the gas and are captured by the drop bin 44. The exemplary emission system 14 uses gravity to drop the particles out of a low velocity gas stream. The drop bin 44 may be integrated with the heating device, or may be separate. The combustion gases, after the second pass 40 b, may then make a third pass 40 c. The third pass 40 c occurs after a vertical 180° turn over the drop bin 44. The cooling of the gas, the vertical climb of the third pass, and the low velocity of the combustion gases contribute to the particulate removal from the combustion gas stream.

Substantially all unburned carbon and residual ash contained in the fuel may be carried over into the second pass of the boiler/hot water heating device where it may be removed in a region of significantly reduced temperature, without the need to shut down the system. Low velocities within the furnace permit small particles to remain in the first pass until the mass is reduced to the point where it can be carried into the second pass. The transition may not occur until the combustion is almost complete. Therefore, the particles passing through the second pass may be predominantly ash (typically alkaline earth metals bound within the wood species). Once the hot gas has completed the second pass, the exhaust gas turns 180 degrees back to the vertical up flow. Between the second and third pass there is a low velocity drop out. This may allow any residual incombustible particulate that is too large to be carried into the third pass to drop out.

An exemplary heat exchange system 42 is a heat exchanger permitting the high temperature gases of combustion to transfer heat to heat exchange media 46, such as another fluid, liquid, or gas. For example, the heat exchange system 42 may be an air to air heat exchanger, Regenerative Thermal Oxidizer (RTO), oven, water heater, or boiler. In one embodiment, the heating device is an oval shaped pressure vessel. The heating device may be low pressure, such as 15 psi working pressure, in order to heat air for residential or commercial space. The heating device is around the combustion area, essentially making a shell around an upper portion of the combustion area. In this application, the entry port of the fuel feed system and secondary air system, as described above, may be substantially tangential to the outer edge of the chamber positioned along the long axis edge of the oval surface. The heat exchange system may be configured, shaped and sized, for the desired application and available space. For example, the heating device may be generally cylindrical, or may be oval to accommodate narrower site locations.

The emission system 14 may further include an emissions control device 48. The emissions control device 48 may be coupled to the system after the third pass 40 c to capture any remaining particulates in the combustion gas stream. This may be a cyclone separator, or an electrostatic precipitator, or other known emissions control system.

The combustion chamber 2 a and the heat exchange chamber 2 b may be integral in that they are portions within the same chamber surrounded by a single continuous unit. Alternatively, the combustion chamber 2 a and heat exchange chamber 26 may be separable to permit easy access to the combustion area to provide maintenance and cleaning within the combustion area, including access to the igniter system.

Referring to FIG. 1B, the combustion chamber 2 a may be sectionally distinct from the rest of the combustion system 10. As shown, the combustion system 10 is housed within a combustion unit 50. Separating the combustion chamber 2 a from the remainder of the combustion unit 50 permits easy access to the combustion space for cleaning and maintenance. In an exemplary embodiment, the combustion chamber 2 a is coupled to the combustion unit 50 by a hinge 52 to permit the combustion space to rotate 54 relative to the rest of the combustion unit 50. Other embodiments of coupling between the combustion chamber 2 a with the combustion unit 50 to permit easy access to the combustion chamber 2 a are also contemplated. One or more of the other systems, including the igniter system 4, primary air system 6, secondary air system 8, and fuel feed system 12, may be mounted to the combustion chamber 2 a so that these systems move together as a single unit from the combustion unit 50 to provide access to the interior of the combustion chamber 2 a. The combustion unit 50 then houses the heat exchange chamber 2 b with the heat exchange system 42 and emission system 14 and any system not mounted to the combustion chamber. For the systems remaining in the combustion unit 50, the combustion chamber 2 a is designed to align with one or more of these systems when it is replaced within the combustion unit 50.

The exemplary combustion system 10 may also include a control system 16. The control 16 system may use a programmable logic controller (PLC) type processor or a printed circuit board type computer/controller to provide necessary air to fuel ratios over a given combustion load. Primary air system 6, secondary air system 8, and fuel feed system 12 may operate at controlled variable settings determined by the PLC's program. Combined with a human machine interface (HMI), the control system will display fuel and combustion levels. The PLC may also control the combustion during start up and shut down mode, making the combustion process fully automated. The control system 16 may include various sensors and feedback systems in order to monitor start-up, shut-down, combustion, fuel feed, and other parameters of the system. Sensor may include temperature sensors within the combustion chamber 2 a and at the igniter system 4; velocity sensors at the primary air system 6, secondary air system 8, and within the combustion chamber 2 a; weight and speed sensors at the fuel feed system 12; composition sensors at the emission system 14.

In an exemplary embodiment, during operation of the exemplary system 10, the control system 16 starts the combustion process. The combustion fuel may be ignited before being fluidized. Thus, the igniter system 4 is turned on to start localized heating. The fuel feed system 12 is started to feed fuel material to the igniter system 4. Fuel is then gravity dropped onto the igniter system 4, including the fluidizing plate 26 from the auger 22. The fluidizing plate 26 at the bottom of the combustion chamber 2 a may retain the combustion fuel within the combustion space and support the igniter system heating element 28 during initial combustion. The primary air system 6 and/or the second air system 8 is started at low capacity to provide sufficient oxygen for ignition, but not sufficient to fluidize the fuel and overcool the fuel before combustion is sustained. The control system 16 may also control the pulsing of the igniter system to reduce or prevent over heating of the igniter. The fuel in contact with the igniter system is heated and begins to smolder and burn. After a period of time, for example, approximately, 2-10 minutes, the control system 16 may determine whether the fuel has ignited.

After ignition, the control system 16 may then initiate the primary air system 6 and secondary air system 8 to a desired level to fluidize the fuel while sustaining combustion. The fluidizing plate 26 permits air to fluidize the fuel from the primary air system 6 and limits the combustion to the combustion chamber 2 a, protecting the igniter system 4. The igniter system 4 is then shut down once sustained combustion is achieved. The control system 16 may then monitor combustion, control fuel feed rates, temperature, air flow, and other parameters of the system. The system may be set by an operator or may be optimized through feedback controls to provide preferred combustion conditions.

During shut down, the control system 16 may control the primary air system 6 and the secondary air system 8 to stop combustion. For example, the control system 16 may set the primary air system 6 and the secondary air system 8 to full or high capacity in order to blow out the combustion fire. The fuel feed system 12 will also stop delivering fuel to the combustion chamber. The entire shut down may take approximately 5 minutes.

While each of the above-described aspects could be implemented in combination, it is also contemplated herein that these aspects be used individually or in sub-combinations. One or more advantages may be realized through implementation of the aspects described herein, including, but not limited to, minimizing maintenance and shutdowns associated with ash removal in a solid fuel fired combustion system; longer igniter life and less wear; improved operator safety during operation by removing the need to remove ash or the igniter system, and higher combustion efficiency without the significant energy required to fluidize sand. Further, according to various embodiments of the combustion system and method, additional benefits may be achieved. For example, the combustion system utilizes a clean combustion space with an up time comparable to natural gas furnaces. Embodiments of the invention utilize fluidizing wood fuel, but without the aid of a fluidizing material, such as sand. This reduces the energy required for combustion. The wood pellets provide the abrasive, in place of the fluidizing material, as it is added to the combustion chamber. The second and third pass embodiments of the system permit unburned organic materials to drop out of the combustion gas before released into the atmosphere, thereby, reducing emissions. The system may achieve 10 hp or approximately 400 kBTU/hour for a system of overall size of approximately 10 feet height.

Although embodiments of this invention have been fully described with reference to the accompanying drawings, it is to be noted that various changes and modifications will become apparent to those skilled in the art. Such changes and modifications are to be understood as being included within the scope of embodiments of this invention as defined by the appended claims. A person of skill in the art would also recognize that embodiments of the present invention may be modified within the scope of the present description. Additionally, various embodiments may include various combinations of the combustion system as described herein. Alternatively, methods involving the combustion system may be run in different order, concurrently, consecutively, etc. and still are within the scope of the present invention.

The invention is primarily described in terms of wood pellet combustion, but other solid fuel materials may also benefit from embodiments of the present invention, and are within the scope of the invention. For example, embodiments as described herein may be beneficial for a nut processing plant in which shells are removed from the nut and the nuts roasted or otherwise processed using heat. The removed shells create a waste product that must be removed from the plant, and the roasting requires energy that must be supplied to the plant. Embodiments of the invention may be used with the removed shells as a solid fuel source to create the energy required for roasting the nut. Accordingly, the previous waste product of the removed shells becomes the energy source to further process the nut. Similarly, other solid biomass materials may find a use for the embodiments as described herein. 

1. A combustion system for use with a pelletized fuel in the absence of a separate fluidizing medium, comprising: a combustion chamber; a fuel feed system coupled to the combustion chamber; an ignition system coupled to the combustion chamber; a fluidizing system coupled to the combustion chamber, wherein the fluidizing system includes a fuel and fluidizing medium formed from a same material; and a heat recovery system.
 2. The combustion system according to claim 1, wherein the fluidizing system further comprises a fluidizing plate at a bottom of the combustion chamber with a plurality of holes to distribute air over the combustion chamber from a primary air system coupled to the combustion chamber through the fluidizing plate.
 3. The combustion system according to claim 2, wherein the fluidizing system further comprises a secondary air system to introduce air into the combustion chamber over the fluidizing plate, wherein the secondary air system introduces air into the combustion chamber off a radial axis of the combustion chamber to promote swirling of combustion gases.
 4. The combustion system according to claim 3, wherein the fuel feed system enters the combustion chamber at a common port as the secondary air system, and the secondary air system introduces air into the combustion chamber at a speed greater than a flame propagation speed to prevent combustion of incoming fuel.
 5. The combustion system according to claim 2, wherein the ignition system comprises an igniter on a first side of the fluidizing plate and a fin on a second side of the fluidizing plate coupled to the igniter through the fluidizing plate, wherein the first side of the fluidizing plate faces away from the combustion chamber.
 6. The combustion system according to claim 6, wherein the fluidizing plate comprises a heat insulating material, the fin comprises a heat transfer material, and the igniter further comprises a pulse cartridge to provide heat to the fin in pulses.
 7. The combustion system according to claim 1, wherein the fuel feed system includes a fuel bin and fuel transport, wherein the pelletized fuel is introduced into the combustion chamber along the fuel transport at a non-zero angle from horizontal.
 8. The combustion system according to claim 1, wherein the combustion chamber is coupled to the combustion system by a hinge to permit access to an interior of the combustion chamber and ignition system.
 9. The combustion system according to claim 1, wherein the heat recovery system comprises a heating device positioned over the combustion chamber to share a common interior space, the heating device including at least three surfaces positioned so that combustion gases contact a first surface of the heating device at a first pass, contact a second surface of the heating device during a second pass, and contact a third surface of the heating device during a third pass; wherein the first surface is an interior surface of the heating device facing the common interior space, and the second and third surfaces are substantially vertical so that the second pass and third pass occur in substantially opposite directions.
 10. The combustion system according to claim 1, wherein the fluidizing medium and fuel are wood pellets.
 11. A method of combusting a pelletized fuel, comprising: Delivering a pelletized fuel into a combustion chamber by a fuel feed system and onto a fluidizing plate; igniting the pelletized fuel with an igniter system coupled to the fluidizing plate; fluidizing the pelletized fuel after combustion is started in the absence of a fluidizing medium beyond the pelletized fuel; and heating a medium in a heat exchanger with combustion gases.
 12. The method according to claim 11, wherein delivering the pelletized fuel further comprises introducing the pelletized fuel into the combustion chamber at a non-zero angle from horizontal through a port in the combustion chamber positioned over the igniter system and below the heat exchanger.
 13. The method according to claim 11, wherein igniting the pelletized fuel comprises utilizing an igniter system including an igniter on a first side of the fluidizing plate and a fin on a second side of the fluidizing plate coupled to the igniter through the fluidizing plate, wherein the first side of the fluidizing plate faces away from the combustion chamber.
 14. The method according to claim 13, wherein fluidizing the pelletized fuel comprises supplying air to the pelletized fuel from a primary air source from under the fluidizing plate, and supplying air to the pelletized fuel from a secondary air source over the fluidizing plate entering the combustion chamber at a non-zero angle from a radial axis of the combustion chamber.
 15. The method according to claim 14, wherein air from the primary air source is passed through the fluidizing plate and contacts the igniter to cool the igniter during combustion, the method further comprising shutting off the igniter after fluidizing the pelletized fuel once combustion is started.
 16. The method according to claim 11, wherein heating the medium in the heat exchanger comprises contacting the combustion gases along three separate surfaces of the heat exchanger during separate passes of the combustion gases, wherein a second surface and third surface are substantially vertical such that combustion gases passing the second surface are traveling substantially downward, and combustion gases passing the third surface are traveling substantially upward.
 17. The method according to claim 11, wherein igniting the pelletized fuel comprises supplying heat in pulses and supplying air to the combustion chamber by an air system at a velocity not sufficient to fluidize the fuel, and wherein fluidizing the pelletized fuel comprises supplying air to the combustion chamber by the air system at a second velocity sufficient to fluidize the fuel.
 18. The method according to claim 11, further comprising extinguishing combustion by stopping the fuel feed system and supplying air to the combustion chamber to cool the pelletized fuel below a combustion temperature.
 19. The method according to claim 11, further comprising rotating a portion of the combustion chamber relative to a remainder of the combustion system to provide access to an interior of the combustion chamber and igniter system.
 20. A method of combusting a wood pellets, comprising: introducing wood pellets into an interior of a combustion chamber at a non-zero angle from horizontal; igniting the wood pellets by heat supplied by an igniter displaced from the wood pellets; fluidizing the ignited wood pellets in the absence of a separate fluidizing medium with a primary air source separated from the ignited wood pellets by a fluidizing plate and a secondary air source entering the interior of the combustion chamber substantially tangential to an edge of the combustion chamber to provide circumferential air flow; passing combustion gases from the ignited wood pellets through a heat exchanger in at least three passes, wherein a second pass and third pass are substantially vertical and the combustion gases are transitioned 180 degrees between the second pass and third pass; and removing particulate from the combustion gases utilizing gravity, a temperature variation between the second pass and the third pass, and the transition of the combustion gases between the second pass and the third pass. 